Cache most recent send-to-editor temp file

Caches the name of the most recently generated temporary file used for
exporting to external editors and uses that file if the processing
parameters are identical and the file exists. This can dramatically
improve speed when exporting to multiple different editors.

rtgui/editorpanel.h

@@ -243,6 +243,9 @@ private:
     Glib::RefPtr<Gio::AppInfo> external_editor_info;
     std::unique_ptr<Gtk::AppChooserDialog> app_chooser_dialog;
 
+    rtengine::procparams::ProcParams cached_exported_pparams;
+    Glib::ustring cached_exported_filename;
+
     class ColorManagementToolbar;
     std::unique_ptr<ColorManagementToolbar> colorMgmtToolBar;